Gutermann's Sew All, locally from Joanns, etc. or my prefered Gutermann's Mara 70...very strong but thin enough to work very well with home machines.
The tension spring.......... Also I have put bobbins in backwards. The way the thread comes off the bobbin is part of the bottom thread tension. Nothing is worse than a bobbin that is wound badly. It will make you question all of your other variables before you find,
that one part of the bobbin thread has somehow crossed under another causing excessive drag.